Histone deacetylases (HDACs) are a class of enzymes that regulate histone acetylation and thus regulate gene expression. HDAC inhibitors have been known to induce cell growth arrest, differentiation, and apoptosis in tumor cells. They have thus attracted great attention as potent anti-cancer agents. See, e.g., Lu et al., J. Med. Chem. 2005, 48, 5530-5535; Kulp et al., Clin. Cancer Res. 2006, 12, 5199-5206; and Ryan et al., J. Clin. Onclo. 2005, 23, 3912-3922.